Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink may be driven toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven from a nozzle toward a medium by an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezo-electric element in the ink nozzle. Electrically-caused distortions of the piezo-electric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in "Output Hard Copy Devices," edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled "Ink Jet Printing").
Ink compositions used in ink jet printers generally comprise deionized water, a water-soluble or water-miscible organic solvent, and a colorant. Generally, the colorant is a soluble dye. Unfortunately, inks comprising soluble dyes can exhibit many problems, such as poor water-fastness, poor light-fastness, clogging of the jetting channels as a result of solvent evaporation and changes in the dye's solubility, dye crystallization, poor print quality including ink bleeding and leathering, poor thermal stability, chemical instability, and ease of oxidation.
Many of these problems can be overcome by replacing the soluble dyes used in the ink formulations with insoluble pigments. In general, pigments have superior properties when compared to dyes, particularly in terms of water-fastness, light-fastness, thermal stability, oxidative stability, and compatibility with both coated/treated and plain papers. However, because the pigments are insoluble in the ink composition and must, therefore, be included as a dispersion, a new set of problems arises. Stability of the ink is critical, both in terms of maintaining uniformity of properties and assuring that pigment does not clog the ink jets during gaps in usage. The ideal, both for stability and cost reasons, of course, would be to utilize the minimum amount of pigment necessary in the ink composition. However, decreasing the amount of pigment in the ink compositions can lead to images having poor optical densities. Further, because of the compositional balancing required to assure the necessary stability, it is also important to assure that the composition has an appropriate viscosity for use in an ink jet printer, as well as good printing properties such as good water-fastness, light-fastness, and minimized running and leathering when applied to the paper. It has been very difficult to develop an ink formulation which optimizes all of these properties. The general approach has been to adjust the dispersants and solvent systems of ink compositions in an attempt to balance and optimize these often competing properties. Prior to the present invention, however, a formulation which optimizes all of these properties had not been achieved without requiring the use of specific polymeric dispersants.
U.S. Pat. No. 5,180,425, Mattick, et at., issued Jan. 19, 1993, describes ink jet ink compositions which include an aqueous earlier medium, a pigment dispersion, and a polyol/alkylene oxide cosolvent. These inks are taught to provide long functional life to ink jet printers and to resist film formation. Lipohic EG-1 is one of the preferred cosolvents disclosed. Other optional solvents which may be used in the described invention include polyethylene glycol.
U.S. Pat. No. 5,302,197, Wickramanayke, issued Apr. 12, 1994, describes aqueous ink jet ink compositions which comprise a pigment dispersion, an aqueous carrier medium, and a cosolvent mixture which includes a polyol/alkylene oxide condensate together with a cyclic amide derivative. Liponic EG-1 is taught to be useful as the polyol/alkylene oxide condensate component.
European Patent Application 603,469, Chan, et at., published Jun. 29, 1994, describes ink jet inks comprising an aqueous carrier medium, specifically-defined alkyl polyol ether cosolvents, and a pigment dispersion (pigment particles stabilized by a dispersant). These inks are taught to provide a good balance of viscosity, surface tension, resistance to nozzle pluggage, print quality, light stability and smear and water resistance. The solvents which may be used in the disclosed invention include 1,2,3-butanetriol.
U.S. Pat. No. 4,597,794, Ohta, et al, issued Jul. 1, 1986, describes an ink formulation used in ink jet printing processes which is said to form a clear image and have good physical properties. Specifically disclosed solvents for use in the ink include polyethylene glycol and 1,2,6-hexanetriol.
It has now been discovered that the use of a very specific cosolvent mixture containing 1,3-propanediol or 1,4-butanediol together with either (a) a low molecular weight polyethylene glycol or related compound or (b) a polyol/alkylene oxide condensate, in an aqueous ink composition containing a dispersion of an insoluble pigment, provides a unique blend of optimized properties. Specifically, these compositions provide excellent properties in terms of stability, optical density (even when low levels of pigment are utilized), viscosity, printing characteristics (water-fastness, light-fastness, minimized leathering, minimized running of ink on the printed page), and printer maintenance problems (i.e., minimized clogging of the printer jets during gaps in printer usage). None of the patents, discussed above, describes or suggests the specific cosolvent combinations of the present invention.
It is, therefore, an object of the present invention to provide an improved pigmented ink composition for ink jet printers which comprises a very specific cosolvent system.
Other objects and advantages of the present invention will become apparent from the following disclosure.